There is mounting evidence indicating that a childhood history of attention-deficit/hyperactivity disorder (ADHD) is associated with an increased risk for psychotic disorders, including schizophrenia. However, the implications of such a history on the symptomatic and functional outcomes of patients with psychotic disorders are still not well documented. This study examined the prevalence of childhood ADHD in patients with first episode psychosis (FEP) consecutively admitted to a specialized early intervention clinic covering a well-defined catchment area, and compared patients with and without a history of childhood ADHD on socio-demographic, clinical, and functional outcomes over a six to twelve months period. Out of 179 patients with FEP, 27 (15%) were treated for ADHD during childhood, consistent with previous literature indicating an association between childhood ADHD and psychosis. FEP patients with childhood history of ADHD had lower academic achievement, earlier onset of psychosis, and higher rates of childhood conduct and learning disorder. While the two groups had similar scores on psychopathology and functioning at baseline, patients with childhood ADHD showed significantly less improvement in positive and negative symptoms, as well as social and occupational functioning. These results strongly indicate that a history of childhood ADHD in FEP is more frequent than that reported in the general population and predictive of poorer clinical response to treatment. This emphasizes the need for actively screening for a history of ADHD in FEP patients and for treatments that are tailored for these patients.